Communication can be seen as a part of the CTDE paradigm as we can still use additional information on the training stage, but now we allow agents to communicate with each other during execution, which is a natural assumption for cooperative environments [1]}, [2]}, [3]} where agents have the same goal and can also be used in mixed environments [4]} to promote cooperation. Despite being considered CTDE [5]}, we argue that communication can greatly benefit from possessing certain properties to ensure decentralized execution. In this paper, we will focus on the two desirable properties of communication that can greatly improve scalability and decentralization of the agents. The first property is called discrete communication and is strongly connected with the limitation of the message channel bandwidth as it restricts agents to sending each other only discrete messages [5]}, [3]}. This property allows us to significantly reduce the number of bits for message transmission and can be considered a universal restriction between the environments on the number of bits allowed for one message. The second property is called locality [8]}, which can be naturally defined in most environments. This property allows an agent to communicate only with its neighbours on the map, which makes the decision making decentralized since each agent takes action individually based only on local information [8]}. Unfortunately, some environments do not exhibit this locality property either because the map is too small or coordination between all agents is required to solve the task.
